In the following discussion, reference will be made to combustibles, combustible gas or a combustion-sustaining gas and it may be advantageous to define the terms which will be used herein so as to avoid confusion. In combustion processes, the term "combustible gas" is intended to refer to a gas stream containing combustible matter such as fuel and includes, inter alia, hydrocarbon gases, gases which entrain atomized liquid fuels and gases entraining particulate solid fuels. A combustible is the burnable substance (gas, solid or liquid) itself. The term "combustion-sustaining gas" is used herein to refer to a gas constituting an oxygen carrier and, more generally, will refer to the mixture of air with recirculated combustion gas, the latter being a gas constituting the products of combustion in a combustion chamber.
In the combustion field there are a number of works on the effects of external recirculation of combustion gases and mixing the recirculated gas with air used as the combustion-supporting gas in order to reduce the partial pressure of the oxygen of the combustion-supporting gas.
This reduction of the oxygen partial pressure results in better utilization of the oxygen and permits burning of the fuel (combustible gas) with only a minor excess of air. It is also recognized that, for effective combustion to proceed, it is necessary to bring about a reaction between the oxygen molecules and the combustible molecules. This means that the ratio of the mass flow of oxygen and the mass flow of combustible gas must lie between two limits, that the absolute flow velocity of the oxygen to be mixed with the combustible gas must not exceed a certain value, and that a source of ignition must be provided.
Thus the reduction of the oxygen partial pressure by the recirculation of the combustion gas for a given mass flow of the combustion-sustaining stream, gives an increase in the interaction between molecules of oxygen and combustibles. This dilution of the oxygen, accompanied by good interaction of the components of the combustible mixture, permits reduction in the proportion of excess air (i.e. air in excess of the stoichiometric requirements for complete combustion) and reducing the speed of combustion of the combustible.
Furthermore, the temperature of the flame is reduced and hence the production of nitrogen oxides can be minimized.
While these advantages are recognized, there are, however, few examples of the use of external recirculation in practice. Thus, in a report presented to the 61st Annual Meeting of the Air Pollution Control Association held at St. Paul, Minn. in 1968, relative to the effect of the recirculation of combustion gases on the emissions generated by the combustion of heating oils, the authors conclude that the benefits obtained as to emission of pollutants, with recirculation burning, can very probably be commercially utilized.
However, in their opinion, it will require a significant development program to transform their prototype into a commercial burner and serious problems of functioning would have to be overcome.
In fact, only several large power plants used in the central production of thermoelectric power utilize recirculation burners recyling combustion gas.
In a number of cases significant noise emission, indicative of unstable combustion, have been noted. This phenomenon is explained by the fact that recirculation supplies to the combustion chamber a non-homogeneous mixture of the air-combustion gas at a macroscopic level.
The importance of the problem of mixing in the phenomenon of combustion is basic. Numerous researchers have investigated and continue to investigate this field. See, for example, the works of Pratt, published in the review "Progress in Energy and Combustion Science" (1975, Vol.1).
According to these works, combustion cannot be effective until the mixture of combustion-sustaining and combustible gases has attained a molecular level which, in turn, does not occur in the absence of a macroscopic mixture.
That which is true for the mixture of combustion-sustaining and combustible gases is equally true for the mixture of air with the recirculated combustion gas.
There are also works which are concerned with coherent structures in turbulence systems (see the article by Davies and Yule of Southampton University, published in the Journal of Fluid Mechanics, Vol. 69, part 3, pp 513 - 537). These works have shown that, in a turbulent system, the flow stream comprises a number of entities or "fluid packets" with coherent structures which are able to subsist in spite of movement of the fluid and its passage through machine elements, tubes etc.
When two fluid streams are brought together, e.g. air and combustion gas, therefore, the aforementioned coherent structures are present as fluid packets of different oxygen concentrations such that the macroscopic mixture, which is at least necessary for the systems of a mixture at the molecular level, is not obtained.
It has already been proposed to subdivide a stream of one of the fluids to be mixed into a plurality of jets at the point at which the first fluid is introduced into the stream of the second fluid. This technique increases the surface of contact between the fluids and creates a certain amount of turbulence. However, heterogeneous pockets of fluids can remain in the resulting mixture. As a consequence the oxygen partial pressure is not uniform from one region of the mixing zone to another but varies significantly at different points of the mixture.
When this mixture is introduced into the combustion chamber in the presence of the combustible, it is noted that the speed of combustion varies as a function of the instantaneous oxygen partial pressure (local oxygen partial pressure) and gives rise to unstable combustion. This appears to have been the reason why the recirculation technique has not been widely utilized heretofore in spite of the advantages theoretically demonstrable and experimentally obtainable with this technique.